A water fluorosensor with a dual detection range for agricultural soil water and trace moisture absorption in organic solvents†

A convenient and sensitive water detection method is critical not only for moisture-free organic synthesis, but also for monitoring the optimum moisture level in soil for maximum agricultural production. A Zn(II)-complex (3), consisting of a phenolic Schiff base ligand (L₁) with two equivalent imine functionalities, detects water in two distinct ranges: ∼0.002–0.5% and ∼0.5–10% (v/v) in various organic solvents (MeOH, EtOH, ⁱPrOH, DMF, DMSO, etc.). When the solvent coordination in 3 is exchanged by water molecules, the photo-induced electron transfer (PET) process in the L₁ moiety is greatly inhibited, resulting in a significant fluorescence intensity increase. As the water reactivity rate is sufficiently slow and follows first order kinetics, a linear increase in the initial fluorescence intensity with increasing water amounts is exploited to detect the higher water%. However, the intensity increase after completion of the reaction exhibited an exponential relationship with water%, and thus the intensity vs. log[water] relation is used to quantify the trace water even at 0.002% (v/v). Because of the high detection sensitivity, we can precisely evaluate even tiny ambient moisture absorption (0.01–0.2% (v/v)) in those organic solvents. Conversely, the detection method based on the initial fluorescence increasing rate is utilized to identify water content (4.2–6.9% (v/w)) in various agricultural soil samples. The operating simplicity, broad and dual detection range, and compatibility with a variety of solvents make it particularly useful for water detection in industry and agriculture.